Other mistake: When Dexter is explaining the contract to Ed, Ed pulls out a pen and clicks it. In the next camera angle, Dexter hands him a pen.
Other mistake: In the scene where Dexter is telling his teacher he can fix the car damage; you see about four teens walking home from school in the background. One of them waves at the camera twice and then walks away.
Other mistake: When Dexter, Ed, and Otis are in the insane asylum, watch Otis's lips as Dexter says, "That means we only got 4 hours to warn 'em." He is mouthing along with Dexter's line. (01:07:00)
Other mistake: When Kurt is putting the shark poison in the sauce look at the color of the poison, it keeps changing colors.
Other mistake: When Mondo Burger opens for the first time, Kurt and his friends walk out to look at Good Burger. Kurt looks down for the actor's mark and puts his head up when he stops.
Other mistake: When Dexter is squirting ketchup and mustard at the Mondo Burger workers he squirts them twice and they are covered with ketchup and mustard.
Other mistake: Ed's golf ball didn't hit anything to make it fly back and hit Roxanne.
Other mistake: Heather the psychopathy lady from Demented Hills who talks to Ed, is at Good Burger in the end of the film. She was supposed to be staying at Demented Hills.
Answer: We don't know that he doesn't. He could simply go to a different school where the school year ends slightly earlier. It's also possible that he's already graduated or dropped out.
LorgSkyegon